Radio frequency identification (RFID) systems typically use one or more reader antennae to send radio frequency (RF) signals to items tagged with RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In response to the RF signals from a reader antenna, the RFID tags, when excited, produce a disturbance in the magnetic field (or electric field) that is detected by the reader antenna. Typically, such tags are passive tags that are excited or resonate in response to the RF signal from a reader antenna when the tags are within the detection range of the reader antenna. One example of such a RFID system including details of suitable RF antennae is described in U.S. Pat. No. 6,094,173, the contents of which are incorporated herein in their entirety. In order to improve the detection range and expand “coverage” it is known to use coplanar antennae that are out of phase. One example of such an antenna is provided in U.S. Pat. No. 6,166,706.
The detection range of the RFID systems is typically limited by signal strength to short ranges, for example, frequently less than about one foot for 13.56 MHz systems. Therefore, portable reader units are moved past a group of tagged items in order to detect all the tagged items since the tagged items are typically stored in a space significantly greater than the detection range of a stationary or fixed single reader antenna. Alternately, a large reader antenna with sufficient power and range to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy and may increase the range of the radiated power beyond allowable limits. Furthermore, these reader antennae are often located in stores or other locations were space is at a premium and it is expensive and inconvenient to use such large reader antennae. In another possible solution, multiple small antennae may be used but this configuration may be awkward to set up keeping in mind that space is often at a premium.
However, use of multiple antennae (or components) has the drawback that multiple transmission cables are used to connect a reader unit to the multiple antennae and/or that the multiple antennae cannot be individually controlled when they are all connected by a single transmission cable to the reader unit.
By way of background, FIG. 1 is a block diagram that illustrates the basics of a prior art RFID system. A reader unit 100 may typically be connected through RS-232 or similar digital communication to a terminal 102 such as a computer terminal. The reader unit 100 is connected by a cable 203 to a reader antenna 200. The reader antenna 200 typically consists of at least a loop 201 and a tuning circuit 202. Although the tuning circuit 202 is shown as a localized part in FIG. 1, one skilled in the art would recognize that it might be distributed around the loop 201. The reader antenna 200 in turn communicates by low power radio waves 105 with one or more RFID tags 1 06 that are typically associated with items, objects (animate or inanimate) or persons that are to be tracked by the RFID system.
The transmission cable 203 is typically characterized by its impedance, which in a simplified form, is approximately the square root of inductance L divided by capacitance C of the transmission cable. For coaxial cables, the impedance is commonly 50 or 75 ohms.
Generally, the transmission cable 203, antenna loop 201, and tuning circuit 202 are connected together in a manner that most efficiently utilizes the RF power at a desired frequency, which for a given RFID system using a loop antenna, such as antenna 200, is typically a “high” frequency such as 13.56 MHz. Another common “low” frequency that is often used for RFID systems is 125 kHz. “Ultrahigh” (UHF) frequencies such as 900 MHz or 2.45 GHz within the RF range are also used with different antenna designs.
A system using multiple antennae powered by a single reader unit and using a multiplexer switch to alternate between the antennae has also been known. Such a system is conceptually represented in FIG. 2 where two separate antennae 200a and 200b are connected to a reader and multiplexer unit 101 through respective transmission cables 203a and 203b. The use of multiple antennae typically improves the spatial coverage when reading tags, without requiring more than one reader unit. The main disadvantage of the arrangement disclosed in FIG. 2 is the need for a separate transmission cable to each of the antennae. Since space is often at a premium, the use of these separate cables is a disadvantage because additional space is needed to install or position each of these separate cables. This disadvantage is accentuated when more than two antennae are used with one reader unit since all of these multiple antennae require separate transmission cables.